A man walks out of a restaurant into the night and sees street lights and brightly lit shop windows. He's so thrilled by the spectacle that he stands there for 10 minutes, just looking. The reason for his joy at such a mundane sight is the fact that he is normally totally blind.

The man is one of nine patients to have been fitted with an Alpha IMS device, the latest retinal prosthesis that can restore sight to blind people.

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The image above is an X-ray of one of the patients, showing the implanted chip with wires running to it from the retina, with a dial behind the ear that can be used to adjust brightness. It is powered wirelessly via a battery in the pocket.

Such devices work only in patients who have lost their vision through diseases such as retinitis pigmentosa that destroy the light-detecting cells in the eye but leave the vision-processing neurons intact. The devices send signals directly to the brain.

But the two devices work very differently. The Argus II converts video from a camera on a pair of glasses into electronic signals "displayed" on a grid of 60 electrodes implanted over a person's retina.

The Alpha IMS, on the other hand, detects light entering the eye instead of using an external camera, which means that a patient can look around by moving their eyes rather than always having to move their head.

It uses a grid of 1500 electrodes implanted underneath, rather than over, the retina: this offers higher resolution. It also makes use of the natural processing power of the neurons in the middle layer of the retina that process motion and contrast. [Proceeding of the Royal Society B]